Superconductive switching path for heavy current

ABSTRACT

A switching path for heavy current includes at least one superconductive winding traversible by a current and switchable from superconductive to electrically normally conductive condition through its intrinsic magnetic field, and magnetic shield means of superconducting material disposed in the vicinity of the winding and positioned in a manner that when the shield means are in superconductive condition, the magnetic lines of force produced by the winding during the passage of current therethrough are forced into a longer path (s1) than without the shield means so that the magnetic field within the winding is smaller than the lowest critical field intensity at any point of the winding, and so that when the current in the winding reaches a predetermined intensity (IO) the shielding effect of the shield means disappears at least partially due to the increased magnetic field whereby the magnetic lines of force are shortened and the magnetic field increases within the winding to a magnitude above the highest critical magnetic field intensity at any point of the winding passed by the current of predetermined intensity, the winding and the magnetic shield means being immersed in a coolant and being coolable thereby to superconductive temperature, the winding being embedded in heat insulating material of relatively high disruptive strength so as to avoid direct contact of the winding with the coolant, the insulating material having a thickness at least on a side thereof facing the coolant which affords recooling of the winding to superconductive condition subsequent to actuation of the switching path.

United States Patent [191 Massar 1 Oct. 23, 1973 SUPERCONDUCTIVE SWITCHING PATH FOR HEAVY CURRENT [75] Inventor: Ernst Massar, Erlangen, Germany [73] Assignee: Siemens Aktiengesellschaft,Berlin and Munich, Germany [22] Filed: July 31, 1972 [21] Appl. No.: 276,305

Primary Examiner-George Harris Attorney-Herbert L. Lerner et al.

[57] ABSTRACT A switching path for heavy current includes at least one superconductive winding traversible by a current and switchable from superconductive to electrically normally conductive condition through its intrinsic v I14!!! 'Ill! A i r 1 (/IIIIIIIIIIIJ'IIIIIIIJ.'; lrllllllllawlllllllt magnetic field, and magnetic shield means of superconducting material disposed in the vicinity of the winding and positioned in a manner that when the shield means are in superconductive condition, the magnetic lines of force produced by the winding during the passage of current therethrough are forced into a longer path (s than without the shield means so that the magnetic field within the winding is smaller than the lowest critical field intensity at any point of the winding, and so that when the current in the winding reaches a predetermined intensity (1 the shielding effect of the shield means disappears at least partially clue to the increased magnetic field whereby the magnetic lines of force are shortened and the magnetic field increases within the winding to a magnitude above the highest critical magnetic field intensity at any point of the winding passed by the current of predetermined intensity, the winding and the magnetic shield means being immersed in a coolantand being coolable thereby to superconductive temperature, the winding being embedded in heat insulating material of relatively high disruptive strength so as to avoid direct contact of the winding with the coolant, the insulating material having a thickness at least on a side thereof facing the coolant which affords recooling of the winding to superconductive condition subsequent to actuation of the switching path.

8 Claims, 5 Drawing Figures I [1/ ll PATENTEDUBIZS ma 3.768.053

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SUPERCONDUCTIVE SWITCHING PATH FOR HEAVY CURRENT The invention relates tosuperconductive switching path for heavy current.

In copending U.S. Pat. application Ser. No. 95,088, filed Dec. 4, 1970 now US. Pat. No. 3,691,491 of which I am one of the joint applicants, there is described a switching path for heavy current comprising at least one superconductive winding switchable from superconductive to electrically normally conductive condition through its intrinsic magnetic field. The switching operation in such a switching path is initiated if a specific critical magnetic field intensity and a corresponding current density are reached when the switching path is subjected to current. The superconductor of the switching path is disposed strip-like so that the intrinsic magnetic field forming in the winding extends parallel to the surface of the strip-like conductor. Switch engineering measures or features in the application or use of such switching paths are described in my article in Elektrotechnische Zeitschrift (Electrotechnical Journal) Edition A, Volume 89 (1968), pages 335 to 339, especially page 338, FIG. 6 and page 339. Difficulties associated with the operation of such switching paths are primarily caused by the fact that often even small differences in the characteristics of the material of the superconductor and in the development of the magnetic field along the switching path, which is many kilometers in length at high voltages, can initially lead to the transition of only isolated locations of the switching path from a superconducting to a normally con ducting condition. More particularly, first those localities of the switching path become normally conducting whose critical magnetic field and critical current density, due to the aforedescribed difference in the properties of the material and the development of the magnetic field are lower or are attained earlier than those of theother localities of the switching path. These isolated localities, which are the first localities of the switching path to change over to electrically normally conductive condition can consequently burn out, leading to the destruction of the entire switching path. This is particularly critical when there is a relatively slow current increase in the switching path, such as when short-circuits are far removed or when individual parts of the system or network are less overloaded, for example.

ln order to assure trouble-free functioning of the switching path, there was proposed in the aforementioned copending application, that magnetic shields of superconductive material be provided in the vicinity of the winding. When the magnetic shields are in superconductive condition, the magnetic lines of force or flux, produced by the winding during the passage of current therethrough, are forced into a longer path than when no magnetic shields are used. Consequently, the intensity of the magnetic field within the winding is lower than the lowest critical magnetic field intensity at any location of the winding. When a predetermined current intensity is attained in the winding, at which the switching operation is to be released, the shield effect of the magnetic shield at least partially disappears due to the increase in the magnetic field which is connected with the current rise in the winding. Because of the resulting shortening of the magnetic lines of force or flux, the magnetic field within the winding increases to a value greater than the highest critical magnetic field intensity at any location of the winding traversed by the predetermined current.

Since the shielding effect of the superconductive shields disappears very rapidly when the critical magnetic field of the shields is exceeded, the shortening of the magnetic lines of force causes the magnetic field in the winding to pass virtually in one jump through the critical range wherein the critical magnetic field intensities are scattered at the individual locations of the winding. The entire winding is thereby very rapidly transferred from superconducting to electrically normally conducting condition, and burn-out of the individual locations of the winding and the consequent destruction of the switching path is thereby prevented.

In the aforementioned copending application, various examples for constructing such switching paths in practice are described. Generally, the switching path conductors are described therein as being applied in the form of bands, strips or tapes of superconductive material, such as niobium especially, on insulating carriers in the form of plates, hollow cylinders or the like. Great care must be taken when winding the superconductor on the carrier in order to avoid damage or tearing. The thickness of the niobium bands or strips is in the order of magnitude of substantially l to 10 m, the width being a few centimeters.

When constructing the switching paths, it is further advantageous to take into consideration that a very rapid temperature increase occurs in the switching path conductor during the switching operation. If one rapidly reaches the range of high specific resistance, the total losses in the switching path conductor until shut off by the mechanical auxiliary switch, then become smaller. Moreover, only little outlay for recooling the coolant is required.

These operating characteristics of superconductive switching paths markedly contrast with those of superconductive windings of conventional construction, because in the latter, as a rule, the most direct contact between the superconductive material and the coolant is striven for. With such a direct contact, the temperature in the switching path conductor in a superconductive switching path can, however, only increase relatively slowly during the switching operation, because the switching path conductor is intensively cooled. lf liquid helium is used as coolant, abrupt vaporization of the helium can occur quite readily, during which a pressure shock or jolt is produced which stresses the entire structure and especially the wall of the cryostat in which the switching path is located. If helium in gaseous form is used as coolant at temperatures slightly above 4.2"K at atmospheric pressure, such shock effects are, in fact, avoided and a less intensive cooling is achieved, yet it is difficult to attain an adequate electrical insulation which reliably withstands the stresses in the windings which occur when traversed by shock waves.

It is accordingly an object of the invention to provide a superconducting switching path for heavy current which avoids the foregoing disadvantages of the heretofore known switching paths of this general type and to provide an improvement in the structure of the superconductor for the switching paths of the aforementioned copending application which affords advantages both in the construction of the switching path as well as in the operation thereof.

With the foregoing and other objects in view, there is provided in accordance with the invention, a switching path for heavy current comprising at least one superconductive winding traversible by a current and switchable from superconductive to electrically normally conductive condition through its intrinsic magnetic field, and magnetic shield means of superconducting material disposed in the vicinity of the winding and positioned in a manner that when the shield means are in superconductive condition, the magnetic lines of force produced by the winding during the passage of current therethrough are forced into a longer path (s,) than without the shield means so that the magnetic field within the winding is smaller than the lowest critical field intensity at any point of the winding and so that when the current in the winding reaches a predetermined intensity (l the shielding effect of the shield means disappears at least partially due to the increased magnetic field whereby the magnetic lines of force are shortened and the magnetic field increases within the winding to a magnitude above the highest critical magnetic field intensity at any point of the winding passed by the current of predetermined intensity, the winding and the magnetic field means being immersed in a coolant and being coolable thereby to superconductive temperature, the winding being embedded in heat insulating material of relatively high disruptive strength so as to avoid direct contact of the winding with the coolant, the insulating material having a thickness at least on a side thereof facing the coolant which affords recooling of the winding to superconductive condition subsequent to actuation of the switching path.

In accordance with another feature of the invention, on the other side of the superconductor, the insulating material also has a greater thickness which is dimensioned primarily with respect to the desired mechanical stability.

In accordance with a further and advantageous embodiment of the switching path, the superconductor is tape-shaped and is mounted on a foil of synthetic material, such as polyethylene terephthalate, for example, by being adhesively secured thereto at separate locations, for example. The superconductor is then covered with an additional foil of synthetic material. Both foils are mutually joined at the edges thereof, for example, by adhesion or welding, so that the superconductor is completely surrounded by insulating material, in accordance with yet another feature of the invention. Both foils of synthetic material are advantageously of different thickness, in accordance with an added feature of the invention.

In accordance with yet another feature of the invention, the superconductor is covered with a layer ofinsulating lacquer and is mounted on a foil of synthetic material. Alternately according to the invention, the superconductor is enveloped or covered on all sides thereof with a layer of insulating. lacquer.

In accordance with a concomitant feature of the invention, which may be desirable for facilitating the manufacture'of the switching path, the tape'shaped superconductor is constructed of a plurality of partial tapes connected electrically in parallel and mounted adjacent one another on a foil of synthetic material and covered with insulating material.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in superconductive switching path for heavy current, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which: I

FIG. 1 is a schematic, cutaway, perspective view of an embodiment of a switching path as shown in the aforementioned copending application;

FIG. 2 is a schematic diagram illustrating the course of the magnetic lines of force in the switching path of FIG. 1 in different operating conditions;

FIG. 3 is a diagrammatic enlarged sectional view of an improved embodiment of the switching path of FIG. 1 according to the invention of the instant application;

and

FIGS. 4 and 5 are fragmentary views of FIG. 3 showing other embodiments of the switching path conductor of the invention.

Reference is now made to the drawings and first particularly to FIGS. 1 and 2 thereof which are shown and described in the aforementioned copending application.

In FIG. 1, the switching path is represented by two elongated windings 1 and 2 having the same winding direction. The windings l and 2 are positioned adjacent each other with their longitudinal axes in parallel, and are electrically connected in series with each other. A shield 3 of substantially large area is provided between the windings 1 and 2. The shield 3 comprises, for example, superconducting sheet metal. The shield 3 extends beyond the ends of both windings 1 and 2.

The windings 1 and 2 comprise tape, band, strip, or the like, shaped superconductors 4 which are wound in single layers on synthetic plates 5 of rectangular crosssection. The individual turns of the windings 1 and 2 enclose rectangular areas having longitudinal sides which are longer than their width. Each rectangular area enclosed by one turn should be as small as possible, so that the inductivity of the switching path may become as low as possible. The shield 3 is rounded off at its free edges 6 by flanging of the sheet or other appropriate arrangements or attachment.

Laterally, the windings 1 and 2 are enclosed as closely as possible by additional shields and form, for example, a closed, quadrangular shaped box 7. The box 7 is shown in cut-away form in FIG. 1. There are free spaces between the two free edges 6 of the shield 3 and the front walls or sides of the box 7, through which the magnetic lines of force or flux produced by the windings 1 and 2 may pass. The other two edges of the shield 3 are preferably affixed to the walls or sides of the box 7.

During normal operation of the switching path, the windings 1 and 2 and the shield 3 are in superconductive condition. A current flowing through the windings 1 and 2 produces a magnetic field which penetrates the windings. For as long as the shield 3 is superconductive, the magnetic lines of force or flux, produced by the windings 1 and 2, cannot penetrate the shield 3, but are forced to follow the paths s], which extend around said shield. FIG. 2 illustrates the course of the magnetic lines of force, in a simplified schematic presentation, which shows said lines of force to be parallel to the shield 3.

When the current flowing in the windings 1 and 2 reaches a predetermined intensity 1, the switching path formed by said windings should transfer from a superconductive condition to an electrically normal conductive condition, abruptly. The windings 1 and 2 are especially rated by a selection of appropriately conducting material so that at the current 1,, the magnetic field, which is characterized by the magnetic flux paths s,, becomes even somewhat smaller with the windings l and 2 than the smallest critical magnetic field at any location of the windings l and 2.

On the other hand, the shield 3 is so rated, by appropriate selection of the superconductive material thereof, that the magnetic field generated by the current l exceeds the critical magnetic field of the shield 3 at the free edges 6. The free edges 6 then lose their shielding effect so that the magnetic lfield may pass through the shield 3. Since the field lines become shorter thereby, the magnetic field is additionally increased and rapidly penetrates the portions of the shield 3 which protrude beyond the ends of the windings 1 and 2. The magnetic lines of force or flux then extend along paths s as shown in FIGS. 1 and 2.

Due to the rapid shortening of the magnetic lines of force of flux, the magnetic field in the windings l and 2 suddenly increases orjumps to a magnitude above the highest critical magnetic field at some point of said winding passed by the current I,,. The critical region within which the critical magnetic field of the switching path varies is therefore passed so rapidly by the magnetic field that the windings 1 and 2 transfer completely from the superconducting condition to the electrically normal conducting condition, and this eliminates burn out of the windings due to premature transition of individual localities of the windings from a superconducting condition to a normal conducting condition.

The shield box 7 prevents, in a superconductive condition of the shield 3, feedbacks of the magnetic lines of force or flux on paths shorter than the paths s,. The box 7 preferably comprises superconducting material having a critical magnetic field intensity which is so high that said box remains in a superconducting condition during the transition of the shield 3 to the normal conductive condition. The entire device is arranged in a cryostat, not shown in FIG. 1, which is filled with a coolant such as,-for example, helium. The walls or sides of the box 7 are provided with openings 8 through which the liquid coolant may penetrate into the interior of said box.

The shortening of the magnetic lines of force which occurs during the disappearance of the shielding effect of the shield 3 is illustrated with particular clarity in FIG. 2. The increase of the magnetic field within the windings l and 2, which is related to the shortening of the magnetic flux or lines of force, may be evaluated in a simple manner. When the total number of turns of the windings l and 2 is equal to w and the windings are passed by the current 1 the following equation defines the magnetic field formed by said winding.

95 H ds l w Immediately before the shielding effect of the shield 3 disappears, the lines of force or flux extend along the path s,. By assuming, as is justified, that for windings which are not too long the amount of the magnetic field H is constant along the path s,, the following equation is obtained.

lH, Is, l w

After the disappearance of the shielding effect of the shield 3, the path s, is replaced by the path s,. The following equation is then obtained.

When the magnetic lines of force or flux pass through or cross the shield 3, the magnetic field in the winding 1 and 2 increases suddenly from the magnitude H, to

The magnitude of the increase of the magnetic field in the windings 1 and 2 is determined by the quotient of both flux paths s, and s That is, the magnitude of the increase of the magnetic field is determined essentially by the fact of how far the shield 3 extends beyond the ends of the windings 1 and 2. The magnetic field in the windings I and 2 is increased more, the further the shield 3 extends beyond the ends of the windings l and 2. If, for example, the paths s is shorter than the path s,, by 25 percent, H equals 1.33 H,, so that H is 33 percent greater than H,. As hereinbefore described, the windings 1 and 2 and the shield 3 are rated so that the lowest critical magnetic field at any point of the wind ing is smaller than H, at the current I,,, but H is greater than the highest critical magnetic field at the current I at any location of the winding. Thus, the range or region wherein the critical magnetic field of the superconductor material of the windings l and 2 may vary lies between H, and H In FIG. 3, there is shown in section, part of a winding of a switching path constructed in accordance with the invention of the instant appliction. A superconductive tape or band 11 of niobium serves as switching path conductor and has a thickness of substantially l to 10 pm, preferably 3 to 5 pm, and is as much as several centimeters wide. The niobium tape 11 is secured adhesively on a band or tape-shaped polyethylene terephthalate foil 12 having a thickness of 10 to 20 am, for example. The niobium tape 11 is covered by another band or tape-shaped polyethylene terephthalate foil 13 which is substantially 6 to 10 m thick, for example. Both foils l2 and 13 are mutually welded at their edges 14 so that the niobium tape 11 is completely enclosed by synthetic or plastic material. The niobium tape 11 enveloped by the foils 11 and 12 is wound in adjacent windings on a carrier 15. Since the superconductive niobium band 11 is mechanically more rugged due to its being enveloped by synthetic material, the winding thereof on the carrier 15 is considerably facilitated and greatly production of the switching stretch is accordingly greatly simplified.

The coolant 16, such as liquid helium, for example, washes around the surface of the thinner tape 13 of synthetic material, and is not in direct contact with the niobium tape 1 1. At the instant the switching path is actuated, the cooling action is therefore largely absent so that a sharp temperature rise occurs in the niobium tape 11. The surrender of heat to the coolant is delayed so that the intermittent or sporadic vaporization which would otherwise occur, is entirely or extensively avoided.

The thickness of the foil of synthetic material surrounding the superconductor must be dimensioned, at least on the side thereof facing the coolant, so that the coolant is at a state for attaining, within the required time for re-switching on the switching path, the recooling of the superconductor to the operating temperature of substantially 4 K, for example. In the embodiment of the invention shown in FIG. 3 wherein the thickness of the foil 13 is suitably dimensioned, the switching path again attains in fractions of a second after a switching operation, the temperature required for the superconductive operating condition of the tape 11.

Regarding this embodiment, an example in terms of numbers is briefly given hereinafter:

Assuming that, upon actuation of the switching path and before the switch-off by means of the mechanical auxiliary switch the niobium tape attains a temperature of about 100 K within about 50 msec. Assuming a mean thermal conductivity of the foil of synthetic material of 6 l cal/cm C sec and taking into consideration that for a thickness of the tape 11 of about 3pm, a thermal content of about 3 10 Wsec per cm of the tape surface must be surrendered to the coolant, one must conclude therefrom that for a thickness of the foil 13 of about 8 pm, the niobium tape 11 can be recooled with high current carrying capacity in the range of superconductivity within about 0.1 see. This period of time is fully adequate to permit automatic rapid reclosing under short circuit conditions wherein reclosing or reconnecting periods of 0.2 to 0.5 seconds are to be expected.

The foil 12 of synthetic material which lies on the carrier 15 is thicker than the foil 13 adjoining the coolant 16 in order to increase the mechanical strength or ruggedness of the switching path conductor which is enveloped by insulating material. By providing intermediate spaces for the coolant between the carrier l5 and the switching path conductor 11 enveloped with insulating material, for example, the form of narrow channels machined in the surface of the carrier, the switching path conductor can be wound on the carrier 15 in such a way that the thinner foil 13 of synthetic material faces toward the carrier 15.

If the speed of recooling plays no decisive role, the possibility then arisesof further increasing the thickness of the envelope of insulating material so that the effects of a switching operation on the coolant are even further reduced. An increased thickness of the insulating material envelope can be of special advantage when it is sought to increase the disruptive strength thereof in order to control shock stresses when traveling waves having a steep front pass therethrough.

In the event, due to the welding of tape-shaped synthetic material foils at the edges thereof or due to the required disruptive strength, the spacing between the adjacent windings of the tape-shaped switching path conductor becomes so great that, because of magnetic field components penetrating between the windings, the current density at the tape edges of the switching path conductor is increased, strips of magnetically conductive, electrically insulating material, for example of ferrite or ferrite synthetic composite material, are disposed parallel to the tape windings at the shock locations between two adjacent windings. In FIG. 3, such strips 17 disposed, for example, sunken in the carrier 15, are shown.

FIG. 4 illustrates in cross-sectional view, another embodiment of the switching path conductor of the invention wherein a superconductive tape 21 is initially surrounded by an insulating lacquer layer 22 and then secured adhesively to a tape-shaped synthetic material foil 23. In such an embodiment, the thickness of the insulating lacquer layer 22, for example, is dimensioned in accordance with the recooling conditions, and the thickness of the synthetic material foil 23 in accordance with the mechanical ruggedness or resistance sought for.

In the embodiment of the switching path conductor shown in cross section in FIG. 5, the tape-shaped superconductor is composed of several adjacent partial tapes 31 to 33 which are connected electrically in parallel. The partial tapes 31 to 33 are adhesively secured adjacent one another on a tape-shaped synthetic material foil 34. Another tape-shaped synthetic material foil 35 covers the partial tapes 31 to 33 and is welded at the edges 36 thereof to the synthetic material foil 34. Such an embodiment is advantageous when the switching path conductor is to have a larger width than that of the individually available superconductive tapes.

I claim:

1. A switching path for heavy current comprising at least one superconductive winding traversible by a current and switchable from superconductive to electrically normally conductive condition through its intrinsic magnetic field, and magnetic shield means of superconducting material disposed in the vicinity of the winding and positioned in a manner that when said shield means are in superconductive condition, the magnetic lines of force produced by said winding during the passage of current therethrough are forced into a longer path (s than without said shield means so that the magnetic field within said winding is smaller than the lowest critical field intensity at any point of said winding, and so that when the current in said winding reaches a predetermined intensity (l the shielding effect of said shield means disappears at least partially due to the increased magnetic field whereby the magnetic lines of force are shortened and the magnetic field increases within said winding to a magnitude above the highest critical magnetic field intensity at any point of said winding passed by the current of predetermined intensity, said winding and said magnetic shield means being immersed in a coolant and being coolable thereby to superconductive temperature, said shield means being embedded in heat insulating material of relatively high disruptive strength so as to avoid direct contact of said shield means with said coolant, said insulating material having a thickness at least on a side thereof facing said coolant which affords recooling of said winding to superconductive condition subsequent to actuation of the switching path.

2. Switching path according to claim 1 wherein said shield means is disposed on a first foil of synthetic niaterial, and a second foil of synthetic material'covers said shield means, both of said foils of synthetic material being mutually connected at the edges thereof.

3. Switching path according to claim 2 wherein both said foils of synthetic material are of different thickness.

4. Switching path according to claim 3 wherein said. shield means is a tape of niobium having a thickness of substantially 1 to m, said synthetic material foil whereon said superconductor is disposed has a thickness of substantially 10 to p.m, and said synthetic material foil covering said superconductor has a thickness of substantially 6 to 10am.

5. Switching path according to claim 1 wherein said shield means is coated with a layer of insulating lacquer and is disposed on a foil of synthetic material.

6. Switching path according to claim 1 wherein said shield means is surrounded on all sides thereof with a layer of insulating lacquer.

7. Switching path according to claim 1 wherein said netic field. 

1. A switching path for heavy current comprising at least one superconductive winding traversible by a current and switchable from superconductive to electrically normally conductive condition through its intrinsic magnetic field, and magnetic shield means of superconducting material disposed in the vicinity of the winding and positioned in a manner that when said shield means are in superconductive condition, the magnetic lines of force produced by said winding during the passage of current therethrough are forced into a longer path (s1) than without said shield means so that the magnetic field within said winding is smaller than the lowest critical field intensity at any point of said winding, and so that when the current in said winding reaches a predetermined intensity (IO), the shielding effect of said shield means disappears at least partially due to the increased magnetic field whereby the magnetic lines of force are shortened and the magnetic field increases within said winding to a magnitude above the highest critical magnetic field intensity at any point of said winding passed by the current of predetermined intensity, said winding and said magnetic shield means being immersed in a coolant and being coolable thereby to superconductive temperature, said shield means being embedded in heat insulating material of relatively high disruptive strength so as to avoid direct contact of said shield means with said coolant, said insulating material having a thickness at least on a side thereof facing said coolant which affords recooling of said winding to superconductive condition subsequent to actuation of the switching path.
 2. Switching path according to claim 1 wherein said shield means is disposed on a first foil of synthetic material, and a second foil of synthetic material covers said shield means, both of said foils of synthetic material being mutually connected at the edges thereof.
 3. Switching path according to claim 2 wherein both said foils of synthetic material are of different thickness.
 4. Switching path according to claim 3 wherein said shield means is a tape of niobium having a thickness of substantially 1 to 10 Mu m, said synthetic material foil whereon said superconductor is disposed has a thickness of substantially 10 to 20 Mu m, and said synthetic material foil covering said superconductor has a thickness of substantially 6 to 10 Mu m.
 5. Switching path according to claim 1 wherein said shield means is coated with a layer of insulating lacquer and is disposed on a foil of synthetic material.
 6. Switching path according to claim 1 wherein said shield means is surrounded on all sides thereof with a layer of insulating lacquer.
 7. Switching path according to claim 1 wherein said shield means is formed of a plurality of partial tapes connected electrically in parallel, said partial tapes being disposed adjacent one another on a foil of synthetic material and covered with insulating material.
 8. Switching path according to claim 1 including strips of magnetically conductive, electrically insulating material disposed at shock locations between two mutually adjacent windings of the shield means and extending parallel to the windings, for conducting a magnetic field. 